Enhanced 911 is a North American emergency response system that enables callers to reach emergency response resources, such as police, fire, and medical response personnel. The location of a caller that made an incoming phone call is automatically determined, and the incoming phone call is answered by a Public Safety Answering Point (PSAP) of a government agency that oversees emergency response in the jurisdiction of the caller. An emergency phone call made using a landline to a PSAP is identified by an associated telephone number of the caller. The telephone number enables the PSAP to use a database to obtain a registered physical address of the caller, and additionally enables PSAP personnel to call back the caller in case of call disconnection.
The U.S. Federal Communications Commission (FCC) also mandates that a wireless carrier network routes 911 emergency calls made using any wireless user device active on the network to a PSAP, regardless of whether the user of the wireless user device is a subscriber of the wireless carrier network. This means that a wireless carrier network is responsible for routing emergency calls made from non-subscriber user devices that are roaming on the wireless carrier network to the responsible PSAPs. However, in cases in which the emergency calls made from non-subscriber user devices are Long-Term Evolution (LTE) calls, a wireless carrier network may be unable to provide the PSAPs with the call back telephone numbers, i.e., Mobile Station International Subscriber Directory Numbers (MSISDNs), of the non-subscriber user devices. As a result, a PSAP may be unable to call back a caller that is using a roaming user device to place the LTE emergency call in case of unintentional disconnect.